The effect of structural defects with the internal degrees of freedom on the parameters of a superconducting material (such as critical temperature ${\mathit{T}}_{\mathit{c}}$, magnetic-field-penetration length λ, and superconductor electron density ${\mathit{N}}_{\mathit{s}}$) is considered using a phenomenological approach. The parameters in question are shown to exhibit local nonstationary fluctuations causing, in particular, a resistance flicker noise peak within the superconducting transition region. The magnetic noise due to fluctuations of λ has been studied as well. Another important factor is associated with local fluctuations of the critical magnetic field that lead to nonexponential magnetic-flux relaxation. It is shown that the states of the fluctuators are dependent on the state of superconducting condensate (e.g., on the presence of a supercurrent which allows some ‘‘magnetic susceptibilities’’ to be assigned to the fluctuators). Thus, in general, one deals with a coupled fluctuators-condensate system, which imposes on the condensate some glasslike features, such as slow relaxation behavior and memory effects. In particular, slow relaxation of the order parameter after a large temperature change has been shown to be expected. As for magnetic effects, a vortex acquires a polaronlike dressing by rearranged fluctuators lowering the critical field, which gives rise to a slow relaxation of the magnetic response. A specific electromagnetic absorption is also considered.

We investigated ${\mathrm{Pr}}_{1.5}$${\mathrm{Ce}}_{0.5}$${\mathrm{Sr}}_2$${\mathrm{NbCu}}_2$${\mathrm{O}}_{10}$ and the superconducting ${\mathrm{Eu}}_{1.5}$${\mathrm{Ce}}_{0.5}$${\mathrm{Sr}}_2$${\mathrm{NbCu}}_2$${\mathrm{O}}_{10}$ materials by several complementary experimental techniques. Oxygen-rich ${\mathrm{Pr}}_{1.5}$${\mathrm{Ce}}_{0.5}$${\mathrm{Sr}}_2$${\mathrm{NbCu}}_2$${\mathrm{O}}_{10}$ is not superconducting and the magnetic-susceptibility data reveal two magnetic anomalies at 11 and 54 K. Both anomalies always appear together and are sensitive to oxygen concentration, whereas for oxygen-poor material both anomalies are absent. The presence of 0.5% Fe dramatically affects only the transition at 54 K and shifts it to 94 K, and Mössbauer studies indicate that this transition is related to antiferromagnetic ordering of the Cu sublattice. The peak in the susceptibility at 11 K is probably due to spin reorientation of Cu moments. While the anomalies are clearly identified by magnetic measurements, no specific-heat anomaly was observed at either temperature. A strong two-dimensional quantum spin fluctuation is assumed as the cause of the unusual behavior at ${\mathit{T}}_{\mathit{N}}$ of Cu. A high linear term of 265 mJ/mol Pr ${\mathit{K}}^2$ is obtained in the specific heat of the oxygen-rich ${\mathrm{Pr}}_{1.5}$${\mathrm{Ce}}_{0.5}$${\mathrm{Sr}}_2$${\mathrm{NbCu}}_2$${\mathrm{O}}_{10}$ material.

Thermally activated dissipation is investigated in epitaxial Bi-2:2:2:3 films in magnetic fields parallel to the c axis. For a wide temperature T and field B range the resistivity ρ shows clear Arrhenius behavior with an activation energy ${\mathit{U}}_0$=A(1-T/${\mathit{T}}_{\mathit{c}}$)/ √B : ρ(B,T)=${\mathrm{\rho }}_0$exp(-${\mathit{U}}_0$/kT)=${\mathrm{\rho }}_0$exp[-A(1-T/ ${\mathit{T}}_{\mathit{c}}$)/kT √B ], which is explained by thermally activated flux motion with the formation of double kinks. Temperature dependence of the irreversibility field ${\mathit{B}}^{\mathrm{*}}$ is predicted by flux-creep theory as √${\mathit{B}}^{\mathrm{*}}$ =C(1/kT-1/${\mathit{kT}}_{\mathit{c}}$), when ${\mathit{B}}^{\mathrm{*}}$ is defined by an electric-field criterion. This relation holds not only for our films but also for magnetically determined ${\mathit{B}}^{\mathrm{*}}$ of Bi-2:2:2:3 samples, which suggests that the irreversibility line originates from thermally activated plastic motion of pinned flux liquid.

${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$${\mathrm{CuO}}_2$ thin films were grown on ${\mathrm{SrTiO}}_3$ (100) substrates by a pulsed-laser-ablation technique. The deposited films were single phase of the infinite-layer structure for 0.15≤x≤0.45. The lattice parameter a decreased with increasing x, indicating p-type carrier doping into the ${\mathrm{CuO}}_2$ plane. In the x-ray-diffraction patterns for these samples, the peak width was quite narrow and Laue functional oscillations were seen around the 001 peak. The peak intensity ratios between 001 and 002 reflections strongly suggested that Li ions were substituted into the Ca ion site of the infinite-layer lattice. Moreover, we observed a lattice image by TEM and determined the composition of a small region by energy dispersive x-ray analysis and true site occupancy by x-ray structural analysis using a four-circle goniometer. The films were found to be almost single crystals without oxygen deficiency.

We propose a microscopic model of a ${\mathrm{CuO}}_2$ plane which includes the essential features of the multiorbital structure of ${\mathrm{Cu}}^{2+}$ ions and preserves the translational invariance of the lattice. We perform the second-order perturbation theory in the hybridization parameter by writing it in terms of a generalized Schrieffer-Wolf canonical transformation and derive the effective Hamiltonian for a p-d spin-spin interaction. The form of this effective Hamiltonian is shown to be modified by the complex structure of ${\mathrm{Cu}}^{2+}$ ions and is directly compared with simpler commonly used models. We emphasize inconsistencies of these models and raise questions on their applicability.

The strong coupling Eliashberg equations for a superconductor stabilized through antiferromagnetic spin fluctuations involve three coupled nonlinear equations for the gap, the renormalization, and the energy shift. Each quantity depends on three variables, namely momentum in the two-dimensional Brillouin zone and on Matsubara frequencies accounting for retardation. Fast-Fourier-transform techniques are applied to generate numerical solutions based on the phenomenological form of the susceptibility suggested by Millis, Monien, and Pines (MMP). The calculated bare susceptibilities are not consistent with the input MMP form, and the unusual temperature dependence of the NMR below ${\mathit{T}}_{\mathit{c}}$ is not reproduced in our calculations.

We present a bosonization method for one-dimensional nonrelativistic fermions with spin. This method is based on the Jordan-Wigner transformation and can be used directly in the strong-coupling limit. We apply this method to the constrained spinon-holon model (at each site we have either a holon or a spinon, in each bond the holon and spinon current vanishes) and show that this model is a Luttinger liquid.

A problem of the irreversible entry of the Abrikosov vortex ring, induced by the self-field of a transport current, into a long type-II superconductor cylinder of arbitrary radius R is solved exactly in the London approximation. The magnetic field and current distribution for the toroidal vortex inside the cylinder are evaluated. The Gibbs free energy of the system is calculated when the transport current is applied. The critical current ${\mathit{j}}_{\mathit{c}}$ of a spontaneous vortex penetration into the superconductor through a surface Bean-Livingston barrier is found to be independent of the radius of the cylinder and close to the depairing current. However, the dependence of the width of edge barrier on the transport current in a thin cylinder is found to differ qualitatively from that in a thick cylinder. As a result of this, in the first case there is no characteristic current but ${\mathit{j}}_{\mathit{c}}$, while in the second case a characteristic current ${\mathit{j}}_{\mathit{c}1}$≃${\mathit{j}}_{\mathit{c}}$/κ arises (κ the Ginzburg-Landau parameter) at which the barrier width drops down to values of the order of the magnetic-field-penetration depth λ, which allows for vortex entry on the surface defects of the size of λ. The latter result is discussed in reference to the high-critical-current observations on the microbridges of high-temperature superconductors.

Neutron-scattering experiments have been performed to study the dynamic spin properties of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$ (${\mathit{T}}_{\mathit{c}}$=33 K) in the temperature range 1.9 K≤T≤150 K and energy range 1.5 meV≤ω≤20 meV; these span, respectively, ${\mathit{T}}_{\mathit{c}}$ and the weak-coupling BCS gap energy 2Δ∼10 meV. The width of S(q,ω) in momentum space decreases with decreasing temperature at fixed energy as well as with decreasing energy at fixed T, becoming very sharp at both low ω and low temperature. It is found that both in the normal state at T=35 K and in the superconducting state at T=10 K the integrated generalized susceptibility χ’’(ω)=${\mathcal{F}}_{(\mathrm{\pi },\mathrm{\pi })}$ d${\mathit{q}}_{2\mathrm{D}}$χ’’(q,ω) is constant above ω∼10 meV with an amplitude about three times that in ${\mathrm{La}}_{1.98}$${\mathrm{Sr}}_{0.02}$${\mathrm{CuO}}_4$ and begins to decrease with decreasing ω at ω∼10 meV, which corresponds to the BCS superconducting gap energy 2Δ. However, we observe nonzero magnetic scattering well below the pseudogap energy at temperatures as low as 2 K. These results are compared with those in ${\mathrm{La}}_{1.98}$${\mathrm{Sr}}_{0.02}$${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.6}$.

Broadening of the resistive transition of sintered ${\mathrm{GdBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ superconductor has been studied varying the measuring current over the range of five orders of magnitude. The observed well-defined branching points in the resistive transition coincide with the quasi-Ohmic saturation of V-I curves and scale with the average grain size. The results suggest the validity of a simple percolation model for dissipation in systems whose global superconductivity is realized by Josephson coupling of the superconducting grains. A common origin of structured resistive transition, observed in many superconducting systems, has been also discussed.

We investigate the physical properties of a three-band generalized t-J model within a systematic large-N slave boson approach allowing for dimer, flux, and uniform magnetic order. The mean-field (N=∞) phase diagrams are characterized by the presence of phase separation, resulting from the competition of kinetic and magnetic energy, which is not spoiled by the inclusion of a direct oxygen-oxygen hybridization term, or of a copper-oxygen short-range Coulombic interaction term, and could be of some relevance in the description of the high-temperature superconducting cuprates. However, such a phase separation takes place between a magnetic dimer insulating phase and a metallic phase with flux order, which is not experimentally observed. The question of the stability of such an exotic phase is addressed in detail showing that the stability region of the flux phase can be greatly reduced especially by removing the nesting in the uniform Fermi-liquid phase and by the effects of the temperature. This scenario leaves open the possibility of an electronic origin of the phase separation observed in the cuprates. We also investigate the effects of the magnetic interaction J on the metal-charge-transfer-insulator transition. Allowing for the fluctuations of the boson fields we show that this transition can still be characterized by the softening of the excitonic mode. The analysis of the Drude spectral weight, the specific heat, the compressibility, and the magnetic susceptibility is finally carried out to leading order in 1/N in the uniform Fermi-liquid phase at zero and low temperature. We calculate the different Landau parameters renormalizing the various quantities.

Vortex-glass superconductivity is studied in an extreme weak-link system of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$/${\mathrm{SiO}}_2$ composite by measuring the I-V curve in a series of low magnetic fields. The feature of the vortex-glass transition, i.e., the changes of the curvature of the I-V curves from the positive to negative are observed and the equilibrium phase boundary line between the normal and superconducting phases in the H-T plane is obtained. In the magnetic field lower than 135 Oe, the system shows a magnetic-field-independent scaling behavior; but a magnetic-field-dependent scaling behavior is observed in higher fields. A relatively low value of the dynamical exponent z suggests that the dynamics of vortex glass in the weak-link network system is different from that in the superconducting single-crystal system.

We present a concept for the search for optical effects which indicates a violation of time-reversal symmetry in high-${\mathit{T}}_{\mathit{c}}$ superconductors. The idea is to investigate the reaction of the material on a magnetic field in a magneto-optical transmission experiment. Below 150 K an annealing field of 400 Oe induces an optical quantity which remains after the field is switched off and which depends on the sign of the field. This signature of broken time-reversal symmetry is confirmed in fields up to 50 kOe. Our results are consistent with a phenomenological theory which takes into account the presence of strains in films. The strains enable a switching of antiferromagnetic domains by magnetoelastic interactions. This interpretation is supported by the dependence of the optical response on ‖H‖.

Infrared pulsed laser irradiation has been used to generate a fast photosignal from epitaxial ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ superconducting films when a small alternating magnetic field is applied in the film plane. No bias current was applied, and no signals were observable in static magnetic fields. The photosignals, 40 ns in duration and tenths of a millivolt in amplitude, are obtained below ${\mathit{T}}_{\mathit{c}}$, with alternating magnetic fields up to 3200 A/m oscillating from 20 to 150 Hz. The laser pulses had a duration of 80 ns, with an absorbed fluence of 1 mJ/${\mathrm{cm}}^2$ at a wavelength of 10.6 μm. The signal is interpreted in terms of flux motion thermally activated by the laser heating. The magnetic force due to the alternating magnetic field exceeds the pinning force following the absorption of the radiation and generates the fast flux motion observed. The signal dependencies on temperature, magnetic-field amplitude, and absorbed fluence, as well as ac-susceptibility measurements, confirm this explanation. It is shown that this thermally activated motion, generated at j∼${\mathit{j}}_{\mathit{c}}$, is faster than an Anderson-Kim creep. We propose that a thermally activated flux-avalanche model based on the self-organized-criticality theory may describe the photosignal observed.

We have measured the optical reflectivity over a broad spectral range of single-phase ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ compounds, and have evaluated the optical conductivity both below and above the superconducting transition temperature. We identify various features in the excitation spectrum of the normal state: a Drude contribution at low frequency, a midinfrared absorption, and several interband transitions. In the superconducting state our results are in full agreement with a BCS singlet ground state, with the measured gap values consistent with the weak-coupling limit. We also evaluate several intrinsic parameters characterizing both the normal and superconducting state, such as the plasma frequency, the relaxation scattering rate, and the mean free path, besides the superconducting gaps and the penetration depth. We compare these quantities with similar ones from other experiments and find satisfactory agreement. Moreover, we present our calculation of the electrodynamic response above and below ${\mathit{T}}_{\mathit{c}}$ with the standard Eliashberg electron-phonon theory of superconductivity, which strongly supports a pairing mechanism mediated by high-frequency intramolecular phonon modes.

We study the dynamics of the sine-Gordon model for the Laplacian roughening transition below two dimensions using the dynamic renormalization-group method. It is found that the dynamic exponent is z=4-ε with ε=2-d, and the mobility scales as ${\mathit{r}}^{\mathrm{-}2.51\mathrm{\epsilon }}$, with changing length r.

In this paper, a simple and uniform method of calculating the zero-field splitting D(T) in the tetragonal phase from the spin-lattice coupling coefficient ${\mathit{G}}_{11}$ in the cubic phase for all ${\mathit{d}}^{\mathit{n}}$ ions in crystals with the perovskite structure has been established. Based on this, the zero-field splitting D(T) for ${\mathrm{Ni}}^{2+}$ ion in the tetragonal phase of a ${\mathrm{RbCdF}}_3$ crystal is calculated by using the parameters obtained from the optical spectra and the isotropic g factor. The result shows reasonable agreement with the experimental findings.

In a previous paper, Zallen [Phys. Rev. 173, 824 (1968)] reported a group-theoretical analysis of the competition between unit-cell complexity and crystal symmetry in determining the presence or absence of infrared-active phonons in an elemental crystal. Here we correct an error in that paper’s treatment of certain hexagonal space groups. Our results modify the minimum-complexity condition for infrared activity: For 228 of the 230 space groups, a necessary and sufficient condition for the existence of symmetry-allowed infrared-active modes in an elemental crystal is the presence of three or more atoms in the primitive unit cell. The two exceptional space groups are P6/mmm (${\mathit{D}}_{6\mathit{h}}^1$) and P$6_3$/mmc (${\mathit{D}}_{6\mathit{h}}^4$); for each of these symmetries, there exists one structure with four atoms per cell and no infrared modes. The P$6_3$/mmc structure includes, as special cases, Lonsdaleite (or ‘‘wurtzite silicon’’) as well as a c-axis-aligned hcp arrangement of diatomic molecules which is relevant to models of solid molecular hydrogen at high pressure.

We study the time dependence of a combined system of an electron described by a tight-binding model interacting with vibrational degrees of freedom in a one-dimensional lattice. The time evolution of our coupled model is described by a nonlinear system of differential-difference equations. Apart from localized solutions (polarons) and extended (Bloch-like) solutions, we found that for a small parameter range, there exists a periodic exchange of energy between the electronic and vibrational degrees of freedom: a small part of the electronic energy is transferred to the lattice and then completely back to the electron with an impressive regularity.

We use the Suzuki-Trotter transformation to map exactly the fully quantum mechanical XY model in two dimensions to a classical system in (2+1) dimensions. In the latter formulation the phase transition is intuitively described and order parameters are introduced. A Monte Carlo study confirms this picture’s transition takes the Kosterlitz-Thouless form. Two additional local symmetries, which have, to date, been neglected in quantum Monte Carlo simulations, are revealed and used.